The use of different metals/alloys, which differ in their metallurgical behavior and which are called in this context “dissimilar metals” in a joint being exposed to high temperature, high pressure loading, high cyclic, and high transients with external forces and moments, can pose problems of stress and reduced lifetime of this joint.
FIG. 1 shows the basic scheme of a combined cycle power plant (CCPP) 10. The combined cycle power plant 10 of FIG. 1 comprises a gas turbine (GT) 11 connected to a water/steam cycle 12 via a heat recovery steam generator (HRSG) 19.
Gas turbine 11 comprises a compressor 14, which aspirates air through an air inlet 13 and delivers the compressed air to a combustor 15, where it is used to generate hot gas by burning fuel 16. The hot gas drives a turbine 17, and the exhaust gas 18 of the turbine 17 passes through the heat recovery steam generator 19 and finally exits as flue gas 20.
Heat recovery steam generator 19 generates steam for a steam turbine 21. In addition, water from the heat recovery steam generator 19 is fed to an air cooler 22 and used to cool down compressed air from the compressor 14, which is fed to the turbine for cooling purposes. While the water is supplied through a water inlet pipe 24, the generated steam flows back to the heat recovery steam generator 19 via steam outlet pipe 23.
A more detailed plan of a combined cycle power plant is shown in document DE 196 45 322, for example.
The high pressure air cooler 22 of GT 11 usually needs to be made of austenitic stainless steel to avoid high temperature corrosion product entering the hot gas path parts of the turbine 17. At the same time, the remaining water/steam side of the plant, which the cooler is connected to, is made of ferritic steel. The weld connection at the steam outlet pipe 23 of the cooler 22 is a dissimilar metal joint or weld of the kind explained above, and thus experiences reduced lifetime issues.
One of the main factors leading to premature failure of such a dissimilar metal connection is the very high temperature gradient in the wall of the pipe during start-up of the plant.